Decreasing water supplies throughout much of the industrialized world has necessitated new methods and systems for utilizing water including contaminants or impurities. Additionally, certain industries have a need for safer, more energy efficient methods and systems for removing water from a target material or solute. Traditional methods and systems of liquid treatment include thermal flash evaporation and membrane filtration. One membrane filtration method is reverse osmosis (RO), in which water is separated from solutes (e.g., contaminants) in a feed liquid by application of a pressure overcoming the osmotic pressure of the feed liquid. For water desalination processes, the pressure sufficient to overcome the osmotic pressure of the feed liquid can be substantial, such as greater than 50 atm, resulting in significant equipment and energy costs. Additionally, the application of pressure often exacerbates reverse osmosis membrane fouling by inorganic and organic molecules.
Forward osmosis (FO) circumvents several of the deficiencies of RO by using osmotic pressure gradients across a semi-permeable membrane to diffuse water from the feed liquid into a draw solution. The draw solution includes a draw solute that provides the draw solution with a greater osmotic pressure than the feed liquid. Draw solutes have included sulfur dioxide, aluminum sulfate, sugars (e.g., glucose, fructose, sucrose), potassium nitrate, ammonium carbonate, and ammonium carbamate. However, use of these draw solutes requires energy intensive processes, such as RO, in order to be removed from the water they draw, or produce materials (e.g., SO2, gaseous ammonia) that are toxic, volatile, and correspondingly expensive to handle.
Switchable polarity solvents (SPSs) have also been proposed for use as draw solutes. When exposed to carbon dioxide (CO2), the SPSs transition between a nonpolar form and a polar form, which have differing solubilities in water. Amines, such as amidines, guanidines, primary amines, and secondary amines, have been investigated as SPSs. However, some of these amines are water sensitive, which reduces their applicability for use in water-containing processes. Highly functionalized amidines and guanidines, as well as tertiary amines, have been found to exhibit an increased compatibility with water. Linear and cyclic tertiary amines including N,N-dimethylbutylamine, triethylamine, N,N-dimethylcyclohexylamine (DMCA), 1-butylpyrrolidine, 1-ethylpiperidine, N,N-dimethylaniline, 4-N,N-trimethylaniline, N,N-dimethyl-o-toluidine, and N,N-dicyclohexylmethylamine have been investigated as SPSs due to their simplicity and low cost. However, not all of these investigated tertiary amines functioned effectively as SPSs.
While DMCA functions as an SPS, DMCA is incompatible with many conventional membrane materials used in RO or FO processes. The DMCA has been found to decompose or otherwise degrade the membrane materials. Furthermore, the transition of DMCA from its polar form to its nonpolar form is incomplete and is insufficient for its complete removal from water, which reduces its effectiveness for feed liquids that contain water.